In various drive tasks, it is desired to operate an asynchronous motor with a rotational speed which is lower as compared with the nominal rotational speed and, in the process, to operate different directions of rotation, without changing the direction of rotation of the feeding network. For this purpose, conventional three-phase power controllers, such as electronic motor switch gear or soft starters, as they are known, can cost-effectively be provided with additional functionalities. Potential applications are, for example, in the positioning of transported goods or in electrically operated gates.
U.S. Pat. No. 4,791,341 A, U.S. Pat. No. 4,524,361 A and U.S. Pat. No. 4,461,985 A already disclose equipping a three-phase power controller with a total of five pairs of antiparallel thyristors. Using this, without changing the direction of rotation of the feeding network, a reversal of the direction of rotation of the connected asynchronous machine can be achieved. Furthermore, it is known from U.S. Pat. No. 4,481,456 A to use a total of nine pairs of antiparallel thyristors in such a way that each external conductor of the feeding network can be connected to each terminal of the three-phase asynchronous machine via a pair of the thyristors. With this arrangement, a matrix direct inverter, as it is known, is implemented.
In addition, EP 0 408 045 B1 and EP 0 512 372 A2 disclose a method and associated devices for motor control in which a pulse pattern is specified, with which fundamental waves can be produced at a frequency which corresponds to 1/(6n+1) of the main frequency of the feeding network, n being a natural number. Here, therefore, fundamental waves can be produced whose frequencies correspond to {fraction (1/7)}, {fraction (1/13)}, {fraction (1/19)} etc., of the main frequency. However, an option to reverse the direction of rotation is not associated with this. Finally, DE 25 58 113 A1 already discloses the proposal of producing 1/(6n+1) of the mains frequency by means of specific pulses. Here, in order to reverse the direction of rotation, either two further pairs of antiparallel thyristors or mechanical changeover devices are needed.